# Structure of the atom

Dr Jeremy Jones et al.

The structure of the atom is key to the majority of the techniques used in radiology, and a general understanding of atomic structure is worthwhile.

The following is an uncomplicated overview of the structure of the atom.

##### Nucleus

The composition of an atom is principally 'empty space'. Its mass is concentrated within the central nucleus which is composed of a specific number of nucleons. Nucleons are either protons or neutrons, and the total number of nucleons is assigned the symbol A (also known as the mass number). The total number of protons within the nucleus is called the atomic number and assigned the symbol Z. Protons and neutrons have a mass of 1. Protons are positively charged, and neutrons have no charge.

• A (mass number): total number of nucleons = protons + neutrons
• Z (atomic number): number of protons

A great many different configurations of the nucleus may occur. In each case, the atomic number defines the element, i.e. it is the number of protons that determines that carbon (having 6 protons) is carbon. However, more than one nuclide may exist for a given element, i.e. carbon may exist with 6 neutrons (carbon-12) or 8 (carbon-14): in each case, there are still 6 protons.

##### Orbiting electrons

Orbiting the positively charged nucleus is a cloud of electrons, which have negligible mass and a negative charge. For an element to have a neutral overall charge, the number of electrons will be equal to the number of protons in the nucleus.

The electrons orbit the nucleus in a similar way to planets orbiting the sun (Rutherford–Bohr model). They orbit in one of the 'shells' that surround the nucleus: these are named K, L, M, N etc. from the centre outwards. Only a certain number of electrons may occupy a particular shell:

• K: 2 electrons
• L: 8 electrons
• M: 8 electrons

In each atom, the outermost shell is called the valence shell and may only be partially full. It has a role to play in the properties that the atom has.

## Article information

rID: 5304
Section: Physics
Synonyms or Alternate Spellings: